1. Field of the Invention
The present invention relates to an optical system capable of efficiently condensing light from a plurality of light sources into an optical element, and a light source module including such an optical system.
2. Description of the Related Art
In the related art, for example, Japanese Patent Gazette No. 3228098, Japanese Patent Gazette No. 2848279, and Japanese Laid-Open Patent Application No. 2002-202442 disclose an optical system that condenses light from a plurality of light sources, such as semiconductor lasers (LD) or light emission diodes (LED), into an optical element, such as an optical fiber. Specifically, in the optical system, light beams from the light sources are converted to a parallel light beam by a collimator lens, the parallel light beam is condensed by a condensing lens having a large diameter, and then the condensed light is directed into the optical fiber.
When it is desired to make the optical system compact by reducing the thickness of the optical system, it may be attempted to reduce the number of the light sources in the optical system arranged in the perpendicular direction, but this results in an increase of the number of the light sources arranged in the horizontal direction in the optical system (for example, the optical system has a rectangular shape).
In order to direct the condensed light into an optical element having a limited NA (Numerical Aperture), such as an optical fiber, it is necessary to limit the NA of the condensed light obtained by the condensing lens to be less than the NA of the optical element, and this requires the NA of the condensed light to be optimized in the horizontal direction, in which way many light sources are arranged. For this purpose, the diameter and focal length of the condensing lens become large, the optical path of the optical system becomes long, and this makes the optical system large. When the focal length of the condensing lens is large, the magnification of the optical system increases, and the diameter of the light beam increases. If the area of the optical element for receiving the condensed light is small, such as the core of the optical fiber, the condensed light cannot be completely directed within the receiving area, and hence the efficiency of light transmission decreases.